The present invention relates to a fuel cell and a manufacturing method therefor. More particularly, the present invention relates to a fuel cell arrangement which is practicably capable, for the first time, of being manufactured in high volume at relatively low cost, and to a high volume process which provides robust and dependable fuel cell construction.
Experimental fuel cells were first produced in the mid-1800's as research expanded in electrochemical storage devices. That early work lead to present-day storage batteries which have, however, progressed relatively little over the past hundred years. When NASA needed a compact, efficient electrical generating system in the 1960's for the U.S. space program, the fuel cell became the energy storage device of choice because cost was not a primary factor.
Generally speaking, fuel cells and storage batteries both produce electricity in essentially the same manner. That is, their oxidizing material (fuel) at a source electrode (cathode) produces positive ions, and reducing material (oxidizer) at the return electrode (anode) produces negative ions. These positive and negative ions combine in an electrolyte forming new stable materials and completing the electrical path.
A battery's source of fuel and oxidizer are the electrode materials which, when depleted, render it inoperative. The electrodes in a fuel cell are, however, permanent structures which provide an electrical path and contribute nothing to the chemical activity. Catalysts initiate the oxidation process which is maintained as the fuel and oxidizer are replenished from external sources. Most present-day fuel cells use hydrogen as the fuel and air as the oxidizer.
An atomic-level, hydrogen permissible filter (Proton Exchange Membrane or PEM) greatly simplifies a fuel cell's structure. In the PEM fuel cell, a hydrogen-rich fluid is fed to the negative electrode (cathode) side of the PEM where a catalyst causes hydrogen atoms to separate from the surrounding fluid material as the electrons of the separated hydrogen atoms are surrendered to the negative electrode prior to passing through the PEM. The oxygen in the air gains electrons through catalytic activity at the positive electrode (anode) side of the PEM. The hydrogen and oxygen atoms (ions) combine, completing the cycle and the resultant is expelled.
The PEM in many fuel cell systems is a thin plastic film commercially available, for example, from DuPont and Gore Industries and similar in handling characteristics to plastic food wrap. Many mechanical difficulties are thus associated with this “plastic wrap”-type PEM. A complex, costly, physical supporting structure is required, including both fuel and cooling-fluid routing.
Electrolyte, PEM and separator treatment demand precision handling which makes final assembly extremely difficult. The process of sealing and connecting the cell stack is the most demanding because the PEM and separator are sensitive to both fluid-wetting and high temperatures. Noble-metal catalysts and easily damaged carbon-compound electrodes contribute significantly to costs, due both to high-priced material and yield losses. The final difficulty is the assembly and sealing of the cells because joining temperatures and pressure must be kept extremely low to avoid destroying the components.
The use of fuel cells for automobiles presents another tremendous challenge. For example, a minimum life expectancy for a family vehicle with only routine maintenance is 100,000 miles over a 5-year span. An automobile must start and operate under a wide variety of adverse conditions, and the drive package must be compact enough to allow placement conveniently away from the passenger compartment and yet be readily accessible for maintenance. The drive package must work safely and start quickly even when abused or slightly damaged. Moreover, there must be compliance with stringent emission standards. From an economic perspective, fuel cells have to compete with current drive-train technology and component replacement, rather than complete-system replacement, is essential.
A tremendous amount of research and development has been devoted to automotive fuel cells. However, prior to the present invention, such R & D has failed thus far to produce economical practicable fuel cell products, because it has concentrated largely on the fundamental scientific principles and basic developments such as perfluorosulfonic-acid based PEMs. On one hand, observed current densities for PEM cells vary from around 25 mA/cm2 to 4000 mA/cm2. On the other hand, achieving such current densities has generally involved using graphite as the conductive electrode material.
Carbon's natural clumping and granular structure present a large porous surface area lattice through which fuel and oxidizer flow. If this large surface area is treated with catalyst material after forming the carbon electrode, large amounts of nobel metal are used. If the carbon is blended with the catalyst and bonding agents prior to forming or attaching as an electrode, the electrical resistance increases. Because of the already-high internal resistance of carbon, any external resistance created at the interface substantially increases the energy loss as heat.
Furthermore, the assembly of PEM electrolyte and carbon electrode cells into usable stacks has proved difficult from the viewpoint of the electrical connections, the fluid seals, and the structure to keep it in place. Also, PEMs rapidly deteriorate as the temperature approaches 90° C. Separate cooling components and complex hydration systems which have been acceptable in the lab are certainly less so in the commercial world. Ceramic electrolyte development has been curtailed due to the perception of high cost and production difficulties.
We have recognized that a key requirement of fuel cells, particularly for automotive applications where cost is a major factor, is that they be mass producible but of consistent high quality, characteristics heretofore unavailable with conventional fuel cells.
In accordance with the present invention, fuel cell fabrication, process and assembly methods are disclosed with the objective of advantageously eliminating many of the components and most of the costly process steps of conventional fuel cell fabrication in order to substantially reduce costs and increase manufacturability without sacrificing the advantages of currently used fuel cells.
Another object of the present invention is to provide a fuel cell in which the mechanical structure, frame and closure is an integral part of the components, and most particularly, the electrodes for use with flexible electrolytes.
Still another object of the present invention is to provide a fuel cell in which the cell-to-cell passages for the fluid inputs and outputs are integrated.
Yet another object of this invention is to provide a fuel cell in which the electrolyte can be placed on either side of the electrode which allows alternate assembly of the electrodes with the electrolytes, negative electrode, electrolyte, positive electrode, electrolyte, negative electrode, etc., and therefore reduces to about one half plus one the number of electrodes needed in current stacked or tandem fuel cells.
Another object of this invention is to provide a fuel cell in which the number of separator plates is reduced for flexible or otherwise difficult to maintain electrolytes or completely eliminated, for rigid structural electrolytes.
Another object of the present invention is the provision of a fuel cell in which the seal or closure at all passages, enclosures, vias and surrounds can be accomplished simultaneously and in any of the known methods such as compressible formed material, adhesives, chemical bonding, eutectic and metal bonding etc.
Another object of this invention is to provide a fuel cell in which the catalytic material can be applied directly to either the electrolyte or the electrode structures using the most inexpensive and reliable known methods such as sputtering, selective plating, chemical vapor deposition, printing etc.
Still another object of the present invention is the provision of a fuel cell in which the electrode electrical connections are externally selectable and connectable to establish the desired electrical power output from interconnected cells.
A further object of this invention is the provision of a fuel cell in which the component alignment advantageously occurs by mechanical design.
A yet further object of this invention is the provision of a fuel cell in which the electrodes are formed equally well by any of several known methods, such as stamping, sintering, casting, molding and multi-layer laminating and etching similar to circuit board technology.
Another object of this invention is the provision of a fuel cell in which the ionization of the fuel and oxidizer is accomplished either at the electrolyte face for cell simplicity or moved some distance to enhance the chemical process and expellation of the spent reaction.
The foregoing objectives for producing, in high volume, fuel cell components which incorporate structure, external electrical connection, internal fuel and oxidizer passage and distribution, exhaust passage and outlet and simple stack alignment assembly have been achieved according to one embodiment in the form of three singular, unitized fuel cell components, namely an electrolyte, a positive electrode and a negative electrode, are complete and ready for stacking, seal or joinery at the exit of their simple process lines. Each of the components works with all of the known electrochemical and electrolyte processes. These components can be stacked to form a complete, alignable, repeatable fuel cell module with internal oxidizer and fuel passage and distribution, internal exhaust passages and external electrical interconnect.
A further object of the present invention is to overcome the current difficulties of electrical interconnect, structural integrity, fuel and oxidizer distribution, maintenance and replacement while produced by a high speed, high volume process such as stamping for metals and suitable plastics and rotary die forming for partially staged ceramics and thermal set polymers.
In one embodiment of the present invention, the electrolyte is produced from a single piece of inert structural material which is processed to allow the passage of only migrating ions, and to which conductive material and the appropriate catalysts are applied to each side to provide a single inclusive unit, i.e. electrolyte, and positive and negative electrodes. Non-conductive fuel and oxidizer distribution plates are added to complete the cell.
The electrolyte is configured such that the main structural component is a peripheral surround of the processing area which is impermeable, includes external electrical connecting tabs, and provides an attachment and seal or closure area. Seal areas are also provided at strategic internal locations for inter-cell fluid passages. The large operating area of the electrolyte is processed to be ion permissive and provides attachment surfaces for conductive electrode and catalytic materials.
In yet another embodiment, the electrode is produced from a single piece of electrically conductive material such that the principal structural component is a peripheral surround of the processing area which is impermeable, includes external electrical connecting tabs and provides an attachment and seal area. Seal areas are also provided at strategic internal locations for inter-cell fluid passages. The large operating area of the electrode is permeable or otherwise open to fluid flow in all directions, transverse, radial and lateral and provides an attachment surface for a catalytic material.
The differences between the positive and negative electrodes are in the location and/or shape of the external electrical connectors and, if applied, specific catalysts. Although it is not necessary to attach the catalysts to the electrode structures, this ability to do so allows the use of less robust mechanical electrolytes which could not support attachment of catalysts to them. The electrodes are configured to have electrolytes placed on either side thereof and reduce volume while increasing fuel cell efficiency.
In yet another embodiment, the electrode is produced from three pieces of material which are separately formed and then joined together to form a unitized structure. The two active pieces are identical, interchangeable, reversible, and are formed and configured by the same processes. Each of the active pieces are produced from a single electrically conductive material such that the principle structural component is a peripheral surround of the processing area which is impermeable, includes external electrical connecting tabs and provides an attachment and seal (closure) area. Seal (closure) areas are also provided at strategic internal locations for inter-cell fluid passages.
The third piece is a distribution plate produced from either a single electrically conductive material which joins the active pieces, thereby producing a common level electrode, or from a single non-conductive material which separates the two active pieces producing two electrically separate electrodes. The distribution plate has a principle structural component constituting a peripheral surround of the processing area which is impermeable, may include external electrical connecting tabs and provides an attachment and seal (closure) area. Seal (closure) areas are also provided at strategic internal locations for inter-cell fluid passages.
In a yet further embodiment, the electrolyte is produced from a single piece of inert non-structural material which is processed to allow the passage of migrating ions in specified regions. The electrolyte is attached directly to one or both of the electrodes as described in the second and third embodiments herein. In one instance the associated catalysts are applied directly to each electrode structure and the non-structural electrolyte attached to one side thereof and a non-conductive alternative or optional seal placed between the electrolyte and electrode assembly and the remaining electrode to complete the cell.
A first modification of the last-mentioned embodiment is the attachment of a non-conductive seal to the non-structural electrolyte. The electrolyte and seal assembly is placed between the appropriate electrodes to complete the cell. In a second modification, the catalysts together with a conductive material are applied directly to each side of the non-structural electrolyte. The electrodes are fabricated with a closed peripheral raised shelf to which is applied a conductive material compatible with those on the electrolyte. This shelf becomes the electrical interface with the conductive catalysts applied to the electrolyte. The electrolyte can be attached to one of the electrodes with the electrode shelf in contact with the associated conductive catalyst. A non-conductive seal is placed between the previously attached electrolyte to electrode and the remaining electrode, with the remaining electrode shelf in contact with the not attached side electrolyte conductive catalyst, thus completing the cell.
Yet another variation is the attachment of a non-conductive seal to the appropriately catalyzed nonstructural solid electrolyte. The electrolyte and seal plate assembly is placed between the appropriate electrodes, with the electrode shelves in contact with the conductive catalysts of the electrolyte to complete the cell.